Prepregs are known as sheet materials made of carbon fibers impregnated with uncured thermosetting resin. In the field of golf club shafts, a plurality of prepregs are wound on a mandrel in the shape of a tapered shaft and thermally cured to be formed into a tapered golf club shaft.
Conventionally, there are usually two types of prepregs: full-length layer and distal-end reinforcing layer. The full-length layer is usually formed into a trapezoidal shape so that the number of turns becomes the same across the full length when wound on a taper-shaped mandrel. The distal-end reinforcing layer is a layer wound only on the distal end portion because the strength (bending rigidity, EI) of the distal end portion becomes insufficient if only trapezoidal prepregs are wound thereon.
FIG. 6 shows a configuration example of a golf club shaft composed of such conventional full-length layers and a distal-end reinforcing layer. This conventional example is made using two trapezoidal bias layers (45-degree layers; the long fiber direction is angled at 45-degrees relative to the shaft axis direction) 11 and 12, each of which is wound two turns (i.e., the number of turns is four in total), three 0-degree trapezoidal layers (the long fiber direction thereof is parallel to the axis of the golf club shaft) 13, 14 and 15, each of which is wound one turn, and a distal-end reinforcing layer 16 composed of a 0-degree layer, in that order from lower layer. The directions of biases (long fibers) of the trapezoidal bias layers 11 and 12 are orthogonal to each other. The distal-end reinforcing layer 16 is a layer for reinforcing the distal end portion and is wound only on the distal end portion. Aside from the distal end reinforcing layer 16, a triangular prepreg 17 composed of a 0-degree layer, which is used to make the distal end portion of the golf club shaft into a straight portion corresponding to the hosel diameter of the golf club shaft, is wound on the distal end portion (on the distal-end reinforcing layer 16).
The trapezoidal layers 11 through 15, the distal-end reinforcing layer 16 and the triangular prepreg 17 which are wound on a mandrel 10 are heated to cure the uncured thermosetting resin of these layers, thereby forming a golf club shaft. Various types of carbon fibers which can be used as carbon fibers of the trapezoidal layers 11 through 15, the distal-end reinforcing layer 16 and the triangular prepreg 17, and various types of thermosetting resins which can be used as thermosetting resin with which such carbon fibers are impregnated are known in the art.    Patent Document 1: Japanese Unexamined Patent Publication H09-131422    Patent Document 2: Japanese Unexamined Patent Publication 2000-51413